1. Field of the Invention
The present invention relates to a current driving system for an electromagnetic load such as a Voice Coil Motor (VCM) used in applications for computer hard disks HDD, stepper motors, etc.
More particularly, but not exclusively, the invention relates to such an amplifier having an input stage that receives a driving signal, a power stage connected downstream of the input stage and connected to the load and an output stage fedback on the input stage to transfer a signal associated to the load, and to a method for driving inductive loads, particularly of the VCM type, wherein a current signal produced by the output of the power stage is applied at one end of the inductive load.
2. Description of the Related Art
As it is well known in this specific technical field, and as shown in FIG. 1, a transconductance amplifier 1 (I/V) typically comprises three main components or blocks: an error amplifier 2, a power stage 3 and a sense amplifier 4 being cascade-connected with each other.
The error amplifier block 2 is generally formed by a fedback operational amplifier, in order to obtain a transfer function effective to ensure a specific system passband.
In applications wherein the electromagnetic load to be driven is represented by an inductor of the VCM (Voice Coil Motor) type, the passband required by the system is around 20-30 KHz in order to ensure small phase-lags for frequencies of some KHz.
The power stage 3 can be of the linear type (in class AB) or of the PWM type (in class D).
The sense amplifier 4 is in turn an operational amplifier but with high CMRR (mainly for driving with PWM signals) and it is capable of outputting a signal that is proportional to the current flowing in the load.
In order to minimize the phase-lag around the most pertinent frequencies also digital solutions have been developed as shown in FIG. 3 wherein the analog error amplifier 2 has been replaced by a digital filter 6 (generally an IIR filter) while the feedback signal has been digitized by using an Analog-to-Digital converter 5 (ADC) downstream of the sensing amplifier 4.
FIGS. 1, 2 and 3 respectively represent:                a known analog solution with a linear power stage;        an analog solution with a PWM power stage; and        a digital solution with a PWM power stage.        
The digital solution, represented in FIG. 3, allows the filter transfer function to be optimized, and it allows the use of external components (resistors and capacitors) to be avoided, which are necessary when the error amplifier 2 is an operational amplifier. Although advantageous, this solution requires however the use of an Analog-to-Digital converter whose cost is not negligible.
The technical problem underlying the present invention is to provide an analog transconductance amplifier (I/V) structure, particularly for driving capacitive loads, having particularly simple and economical functional and structural features and so as to allow a high passband but void of external components for loop compensation.